<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<style>
    div{ width:40px; height: 40px; border:1px solid;}
</style>
<div></div>
<p> <button id="btn">change</button> </p>
<script>
    var btn = document.getElementById('btn');
    var div = document.getElementsByTagName('div')[0];
    btn.onclick = function () {
        div.style.border = 'double red';
    }
</script>
</body>
</html>